1. Field of the Invention
This invention relates to a method and apparatus for adjusting the spectrum shape of a speech signal to enhance the speech quality of the decoded speech and synthesis speech.
2. Description of the Related Art
In a speech encoding/decoding system for encoding a speech signal at a low bit rate, supplying the coded data to a transmission system or storage system and then decoding the coded data, a post filter is disposed on the final stage of the speech decoder in many cases in order to enhance the subjective speech quality of the speech signal decoded and reconstructed on the decoding side.
In the conventional post-filtering speech decoding apparatus having a post filter incorporated therein, various parameters contained in coded data are decoded by a parameter decoder and a speech signal is reconstructed by a speech signal reconstructor based on the decoded parameter information.
The post filter is arranged on the succeeding stage of the decoder having the parameter decoder and the speech signal reconstructor. The pitch filter is constructed by cascade-connecting a pitch harmonics emphasis filter, spectrum envelop emphasis filter, high-pass filter and gain controller.
The function of the post filter is roughly divided into emphasis of pitch harmonics, emphasis of spectrum envelop, emphasis of high-pass component and filter gain control. Among the above factors, the pitch harmonics and spectrum envelop are important factors for determining the tone and phoneme of a speech and a clear speech which sounds free from noise can be created by emphasizing these factors. The filter gain control is necessary to keep constant the level of the speech signal at the time of input to and output from the post filter.
Emphasis of high-pass component is effected to compensate for the insufficient quality of the high-pass component of the speech caused by the characteristic of the post filter and coding such as "muffled speech sound quality" and "less-audible speech sound quality". Particularly, the filter used for emphasis of spectrum envelop tends to have an unnecessary spectral tilt (tilt of low-pass emphasis on average) in many cases and the emphasis of high-pass component is used to compensate for the above tendency.
In the prior art, as the high-pass emphasis filter, for example, a filter having a fixed transfer function of C(z)=1-.mu.z.sup.-1 (.mu. is a fixed value of approximately 0.4) is used. If the above high-pass filter is used, the "muffled speech sound" can be improved and the subjective sound quality can be enhanced to some extent. However, for example, a speech in an interval such as a consonant interval which does not require the high-pass emphasis will be subjected to excessive high-pass emphasis to produce abnormal sound in the high frequency domain, and as a result, sufficient improvement of sound quality cannot be attained.
That is, by carefully listening to and analyzing the muffled speech sound, it is understood that the speech is not always muffled and the speech sounds muffled as a whole since the time length of the speech interval in which the high frequency sound is not fully produced is long. The degree to which the high frequency sound is not adequately produced is different for each speech interval. Therefore, if the high-pass filter having the fixed transfer function is used, the interval in which the high frequency sound is adequately produced is also subjected to high-pass emphasis, thereby deteriorating the sound quality.
As another prior art, a method for subjecting the transfer function F(z) of the spectrum envelop emphasis filter to predictive analysis and adequately changing the value of a parameter .mu. in the transfer function C(z) of the high-pass filter based on the result of predictive analysis is known. However, in this method, since the transfer function F(z) of the spectrum envelop emphasis filter is represented by that of a pole-zero filter whose order is generally high, the calculation for deriving the parameter .mu. becomes extremely complex.
As described above, the conventional post filter using the high-pass filter with a fixed transfer function has a problem that a speech in an interval which does not require the high-pass emphasis will be subjected to excessive high-pass emphasis to produce abnormal sound in the high frequency domain, and the post filter for predicting the transfer function of the spectrum envelop emphasis filter and adequately changing the transfer function of the high-pass filter based on the result of prediction has a problem that the amount of calculations becomes extremely large.